1. Field of the Invention
This invention pertains to video circuits that incorporate line delays, and in particular to circuits that average several video lines or compensate for dropouts in the reproduced signal.
2. Description Relative to the Prior Art
A new video recording format has been proposed in recent years in which a small, flexible magnetic disk is used to store up to 50 still video pictures. This format provides the option of storing a full frame (two circular tracks per picture) for 25 pictures, or a single field (one circular track per picture) for 50 pictures. Because of the resolution limitation of a typical video source (such as the solid state sensor in an electronic still camera) sufficient picture information for a full frame may be difficult to obtain. A single field, consequently, is often all that is available for recording on the disk for each picture.
The recorded disk is inserted into a video disk player, which converts the video signals recorded on the disk into an NTSC television signal. In order to provide a signal for a standard television receiver, it is desirable to form a second field with at least the luminance of the second field interpolated from successive lines of the recorded (first) field. For this reason, it is known to delay (during first-field-playback) the lines of the first field by a half-line (to obtain the second field) and then to average the amplitude of the picture element video signals on two successive lines (see, for example, U.S. Pat. No. 4,470,076). As shown by this patent disclosure, luminance averaging requires a one-line delay in order to simultaneously process two lines of signals.
A dropout compensator is useful in a video disk player for replacing information found to be missing after the video signal has been recorded and reproduced from a magnetic disk. A dropout, which is the designation given to the missing information, may be due to imperfections in the magnetic disk or to problems in the recording and/or reproduction process itself. When the dropout occurs, it causes a disturbance in the video signal and, usually, manifests itself in the form of a streak or flash on the screen of the television receiver. Dropout compensation depends on line-to-line redundancy in the picture information, thus making it possible to compensate for a dropout by substituting information from a spatially-corresponding part of a previous line. In doing this, a dropout compensator ordinarily stores information from a previous line and inserts the stored information when a dropout occurs.
Storing the video information for dropout compensation, just as averaging two lines of video information for a second field, requires the use of a one-line delay element (see, for example, U.S. Pat. No. 2,996,576). It is informative to realize that the aforementioned video player is likely to be a cost-sensitive consumer product. Delay elements, like other parts of the player, represent cost elements that need to be carefully trimmed. While anyone can see that, from this perspective, one delay element is to be preferred over two, the combination of the two heretofore disparate functions of dropout compensation and line averaging is not evident from the prior art. A dropout compensator ordinarily switches between two line signals: one signal is a freshly reproduced signal . . . which may include a dropout . . . and the other signal is a one-line delayed signal free of dropouts. Though a line averager also uses two line signals, one delayed by a line with respect to the other, it is highly desirable that both signals be free of dropouts.